The invention relates to a method for exciting transverse magnetization in NMR-experiments by irradiating the nuclear spin-system, which is subjected to a magnetic field of high field strength, with a sequence of RF-pulses to provide for a--substantially--phase-distortionless excitation followed by a free induction decay resulting in a spin-echo signal used for further processing and evaluation in terms of the physical quuantities or information of interest, said sequence of RF-pulses comprising a first RF-pulse providing for a 90.degree. -flip, and a second RF-pulse, providing for a 180.degree. -flip of the magnetization, each around an axis which is orthogonal to the direction of the magnetic field, the second RF-pulse being generated after elapse of a defocusing time interval .tau. following the first RF-pulse, as well as to NMR-spectroscopic devices which are operated by using the method according to the invention. A method of the above mentioned kind is known from the scientific publication by R. Freeman, S. P. Kempsell and M. H. Levitt, J. Magn. Reson. 38, 453 (1980).
According to the known method, the transverse magnetization is excited by a "spin-knotting sequence" consisting of a group of three pulses, a 10.degree. (X) pulse, a 60.degree. (-X) pulse, and a 140.degree. (X) pulse, separated by suitable intervals, seen in a reference frame, rotating about the Z-axis in synchronism with the transmitter frequency, where the static magnetic field is directed along the Z-axis, and the transverse magnetization aligned along the Y-axis corresponds to an absorption mode signal.
A somewhat larger bandwidth of the spectral range within which transverse magnetization can be excited is obtained by pulse modulation of a fixed frequency carrier in combination with phase modulation (R. Tycko, H. M. Cho, E. Schneider and A. Pines, J. Magnetic Reson. 61, page 90, 1985), however, the improvement resulting therefrom is not of substantial importance.
Compared with usual practice to excite transverse magnetization in a NMR-experiment by merely applying a 90.degree. pulse, the known methods provide for a considerable improvement with respect to phase-distortionless excitation. The bandwidth of the spectral range within which excitation of transverse magnetization is possible is, however, a comparably small interval, and the known methods, therefore suffer from the disadvantage that they are not effective to cover spectral ranges that are larger than the RF-amplitude, expressed in terms of the frequency .omega.=.gamma.B.sub.1 which is equivalent ot the field contribution of the exciting RF.
The same holds for the situation prevailing in one- and multi-dimensional Fourier-Spectroscopy (R. R. Ernst, G. Bodenhausen and A. Wokaun, "Principles of Nuclear Magnetic Resonance in One and Two Dimensions", Clarendon Press, Oxford, 1987) according to which excitation in a frequency range of limited band-width is obtained by pulsed time modulation of a monochromatic transmitter-frequency.
On the other hand as it is well known, in continuous-wave (CW) spectroscopy (R. R. Ernst, Adv. Magn. Reson. 2, 1-135 (1966) it is possible to sweep the exciting transmitter frequency over spectra with arbitrary width. CW-spectroscopy, however, requiring a "slow" frequency-sweep suffers from the disadvantage of very long measuring times and, therefore, tends to be largely supplanted by Fourier-Spectroscopy.
If, to the object of reducing measuring time, continuous-wave spectra are recorded with a moderately fast frequency-sweep, one observes so-called "wiggles" in the spectra. In favourable cases, these artifacts my be removed by deconvolution (J. Dadok and R. F. Sprecher, J. Magn. Reson. 13, 243-248 (1974)), a method which has been called "Rapid Scan Fourier Transform Spectroscopy, by Gupta et al. (R. K. Gupta, J. A. Ferretti, and E. D. Becker, J. Magn. Reson. 13, 275-290 (1974)), since Fourier Transforms may be used very effectively to simplify the convolution integrals. In contrast to pulsed Fourier-Spectroscopy, however, the signal is recorded while the exciting RF-field is swept through the spectrum, exactly like in CW-spectroscopy.
A completely different approach to rapid-scan spectroscopy has been introduced by Delayre (J. Delayre, U.S. Pat. No. 3,975,675), according to which the magnetization is first excited by a frequency-swept pulse, a so-called "chirp pulse", as it is well-known per se in ion cyclotron resonance (ICR), where chirped pulses are employed in routine measurements (M. B. Comisarow und A. G. Marshall, Chem. Phys. Lett. 26, 489 (1974)), and their use has recently been extended to two-dimensional ICR spectroscopy (P. Pfandler, G. Bodenhausen, J. Rapin, M. E. Walser and T. Gaumann, J. Am. Chem. Soc. 110, 5625-5628 (1988)). In Delayre's experiment, in contrast to rapid-scan spectroscopy, the free induction decay is recorded after the end of the exciting chirp pulse. Due to the large frequency-dependent phase errors in the resulting spectra, Delayre's approach has never enjoyed much popularity in NMR.